Arc chute



March 22, 1955 R. A. NELSON 2,704,381

ARC CHUTE Filed Sept. 50, 1952 Inventor- Russell A. Nelson,

His Attorney.

United States Patent ARC CHUTE Russell A. Nelson, Norristown, Pa.,assignor to General Electric Company, a corporation of New YorkApplication September 30, 1952, Serial No. 312,251 12 Claims. (Cl.18-475) The present invention relates to are chutes or shields commonlyused in blowout type circuit breakers, such as magnetic or gas blastcircuit breakers, for the purpose of confining and extinguishing the aredrawn between the opening electrical contacts of such devices. It isparticularly concerned with an improved inorganic molding compositionfor the manufacture of such are chutes.

In blowout type circuit breakers, the arc chute is so designed that thearc will be confined in a relatively small space during theextinguishment thereof, and it is, of course, important that the chutebe made of some areand heat-resistant material which will aid in theinterruption of the circuit by cooling the arc'to extinguishingtemperatures. The design of a typical chute is shown in the accompanyingdrawing, in which Fig. 1 illustrates a circuit breaker in partialsectional view including an arc chute of the type with which the presentinvention is concerned; Fig. 2 is a cross-sectional View taken alongline 22 of Fig. 1; Fig. 3 is a view taken along line 33 of Fig. 1; andFig. 4 is a view in cross section of a slab or block of moldedheat-resistant insulating material which, by suitable machining, isconverted to a chute of the desired shape and size.

In Fig. 1 there is illustrated a circuit breaker comprising an arc chute1 which in the circuit breaker is associated with a pair of relativelymovable contacts, not shown. Relative movement between these contacts isobtained by means of an operating mechanism generally indicated at 2.The are chute 1 is in two sections which together form spaced side walls3 with each side wall having on the side facing the other wall aplurality of parallel fins 4 integral with the side wall. The sectionsare suitably clamped together by means of members 5 and bolts 6 to forma chute-like structure with the fins 4 extending perpendicularly intothe chute opening in staggered interleaved relation with respect to eachother to form an extended sinuous or zigzag passageway for the are as isbest shown in Fig. 2 of the drawing. The detailed operating mechanism ofthe circuit breaker is not part of the present invention and, therefore,has not been particularly described herein. For such details, referenceis made to Patent 2,293,513-Linde as describing a representative circuitbreaker in which the chutes of the present invention are employed.

A preferred material for the manufacture of arc chutes for electric aircircuit breakers is of the type described and claimed in Patent2,366,485-Brink et al. The insulating material is an inorganiccomposition essentially comprising concentrated (at least 60%, andpreferably 75%) ortho-phosphoric acid and asbestos of the chrysotiletype. In addition to the asbestos and phosphoric acid, the compositionmay also contain substantial quantities of zircon, talc or other filler.

As described in the Brink et al patent, the asbestosphosphoric acidcompositions can readily be hot-molded. During the molding, there is areactlon between the phosphoric acid and the asbestos so that the moldedproduct can be extracted from the mold. In the manufacture of the largechutes, it has been common practice to hot-mold the asbestos-phosphoricacid compositions into the form of slabs or blocks approximating theshape of the final chute section and thereafter to machine the slabs tothe desired shape. For example, the asbestosphosphoric acid compositionsare ordinarily molded mto the form of a block or slab having theapproximate cross section shown in Fig. 4 of the drawing. Such a slab iscomposed of a thinner section 8 and a thicker section 9. Afterhot-molding, a plurality of grooves are cut in the thicker portion 9 ofthe slab at spaced intervals to form the fins 4 of the arc chutesection. The machined articles are then subjected to a heat treatmentfor the purpose of further removing any water from the molded productand completing the cure or hardening reaction between the asbestos andphosphoric acid. Such a heat treatment is ordinarily carried out attemperatures from about 250 to 700 F., preferably about 600 F., for aperiod of time ranging from about 8 to 12 hours. Finally, it is also thepractice to flame condition the finned surface of the chute exposed tothe arc in order to partially vitrify or glaze the machined areas andthereby improve the arc interrupting properties of the chute. The flameconditioning treatment ordinarily comprises playing an oXy-acetylene orother high temperature flame over the surface of the chute until aglazed or fused film is obtained.

In practice, it has been found that arc chutes prepared by this processhave exhibited considerable warpage during the baking cycle followingthe machining operation and to a greater extent during the flameconditioning process. Considering the size of the chute sections, it isquite obvious that any significant warpage will prevent proper alignmentof the arc chute sections when clamped together in the form shown inFig. 2 of the drawing.

The present invention is based on the discovery that the warpage problemcan be entirely eliminated or substantially alleviated by including inthe composition from which the chutes are molded from about 3 to 10%, byweight, of an organic ester of orthophosphoric acid based on the weightof the composition. While the effect of the organic phosphate is notcompletely understood, it has been found that its use has reduced lossesdue to warpage during baking of the machine chutes by an average of 93%while a further decrease in warpage is also obtained during the flameconditioning process. The phosphate ester does not affect the final useof the material as an arc chute or are interrupter.

Tricresyl phosphate is a preferred phosphoric acid ester for thepractice of the present invention. However, other organic esters ofortho-phosphoric acid may also be employed in place of or in combinationwith the tricresyl phosphate. Such esters have the general formula R OR'-O=O where at least one and preferably three of the radicals R, R andR" are the same or different hydrocarbon radicals, such as alkyl, aryl,alkaryl, aralkyl and alkoxyallyl, etc. radicals and the remainder, ifany, are hydrogen atoms. Examples of such compounds are tri-ethylphosphate, tributyl phosphate, tri-Z-ethylhexyl phosphate,tributoxyethyl phosphate, tri-phenyl phosphate, cresyl diphenylphosphate, tri-dimethyl phenyl phosphate, diphenyl mono-o-xenylphosphate, tri-ptert-butylphenylphosphate, mono-cresyl phosphate,di-cresyl phosphate and the like.

The advantages of the present invention will become more apparent whenit is considered that chutes 20" in length molded from a compositionconsisting of 52 parts zircon, 35 parts asbestos and 18 parts of 75%phosphoric acid will, on the average, show a warpage of about 81) milsover the 20" length during the baking cycle following the machiningoperations, and an additional 180 mils during the flame conditioningtreatment. However, the addition of 4 /2%, by weight, of tri-cresylphosphate to this molding composition before molding reduced the averagewarpage during the baking cycle following the machining operation toless than 20 mils and the additional warpage after the flame treatmentto an average of about mils.

The invention is, of course, not limited to any specific phosphoricacid-asbestos composition and zircon, talc and other inert fillers maybe included along with the asbestos and phosphoric acid. However, it isdesirable for best results that the phosphoric acid and asbestos bepresent in certain proportions. The preferred molding compositions willordinarily contain about 1 part, by weight, of concentratedortho-phosphoric acid for each 1.7 to 3.5 parts, by weight, asbestoswith the optimum ratio depending to some extent on the texture of theasbestos, i. e., the fiber diameter and length, and the nature of anyfiller which may be included in the composition. Zircon is a preferredfiller and may be present in amounts from to 4 parts, by weight, zirconfor each part asbestos. Talc or other fillers may replace part or all ofthe zircon. In general, the phosphate ester, because of its cost, is notemployed in amounts substantially exceeding about by weight, of themolding composition.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A hot-molding thermosetting composition comprising a reaction mixtureof chrysotile asbestos, concentrated ortho-phosphoric acid having aconcentration of at least 60% and from 3 to by weight, of a phosphateester.

2. A molding compound containing, by weight, about one part concentratedortho-phosphoric acid, from 1.7 to 3.5 parts chrysotile asbestos, and 3to 10%, by weight based on the combined weight of acid and asbestos, ofan organic ester of phosphoric acid.

3. The molding compound of claim 2 in which the organic ester ofphosphoric acid is a tri-aryl phosphate.

4. The molding compound of claim 2 in which the organic ester ofphosphoric acid is tri-cresyl phosphate.

5. A thermoset molded product comprising a heat and pressure reactedmixture of chrysotile asbestos fibers, concentrated ortho-phosphoricacid and from 3 to 10%, by weight, of an organic ester of phosphoricacid.

6. The thermoset molded product of claim 5 in which the organic ester istricresyl phosphate.

7. An are chute for air circuit breakers comprising the molded productof claim 5.

8. An are chute for air circuit breakers comprising the molded productof claim 6.

9. A thermoset molded product produced by reacting under heat andpressure a mixture of chrysotile asbestos fibers, zircon, concentratedortho-phosphoric acid and from about 3 to 10%, by weight, tri-cresylphosphate.

10. In the manufacture of an arc chute by a process which compriseshot-molding a reaction mixture of chrysotile asbestos and concentratedphosphoric acid and machining one surface of the molded body to formthereon a plurality of spaced projections followed by a baking of themachined product at an elevated temperature, the step of including inthe reaction mixture from 3 to 10% of an organic ester of phosphoricacid for the purpose of preventing warpage of the machined productduring the baking step.

11. The method of alleviating warpage of a machined hot-molded reactionmixture of ingredients essentially consisting of chrysotile asbestos andconcentrated phosphoric acid during the subsequent baking of themachined product at elevated temperatures which comprises the step ofincluding in the hot-molded mixture from 3 to 10%, by weight, of anorganic ester of phosphoric acid.

12. In the manufacture of arc chutes by a process which compriseshot-molding a composition essentially comprising chrysotile asbestos andconcentrated orthophosphoric acid into the form of a slab, machining theslab into the form of an arc chute having a plurality of fins extendingfrom one surface thereof, baking the machined product for the purpose ofremoving moisture therefrom and finally playing a flame on the machinedsurface to obtain a glazed surface coating thereon, the improvementwhich comprises including in the hot-moldcomposition from 3 to 10%, byweight, tri-cresyl phosphate for the purpose of alleviating warpageduring the baking and flame treating operations.

References Cited in the file of this patent UNITED STATES PATENTS2,220,019 Morgan Oct. 29, 1940 2,323,951 Wainer July 13, 1943 2,366,485Brink et a1. Jan. 2, 1945 2,383,812 Navias Aug. 28, 1945 2,479,504 Mooreet al. Aug. 16, 1949

